The general principle of solar thermal technology is based on the concept of the concentration of solar radiation to produce steam generally, which is then used in standard power plants.
One of the greatest challenges in the development of solar thermo power plants is to increase the capture of solar energy, which has a relatively low density. There are two types of solar concentrators: line-focus concentrators and point-focus concentrators. The line-focus concentration is easier to install, since it has fewer degrees of freedom, but it has a lower concentration factor and therefore it reaches lower temperatures than point-focus concentration technology.
That is why the aim is to continue progressing in the development of the receiver tubes used in line-focus concentrators, to increase the capture of solar energy and to reduce thermal losses, such that the overall performance of the concentration plant increases.
The present invention revolves around one of the elements that is a part of said receiver tubes, specifically it deals with an insulating element for an expansion compensation device, which is explained below.
In general, a receiver tube consists of two concentric tubes between which a vacuum is produced. The inner tube, or absorber tube, through which the fluid that is heated flows, is made of metal and the outer tube is made of glass, usually borosilicate.
The expansion compensation device is placed between both tubes, in such a way that it allows movement in the longitudinal direction of the tubes and ensures the vacuum, absorbing any strain that the existing differences between the metal and the glass expansion coefficients might create.
However, this element needs to be insulated at its extremity in order to avoid losses that can take place due to natural convection. This insulating element is the one being developed in this invention.
In the state of the art there are known several solutions for this element, but the element that provides best results is the one disclosed by SCHOTT in the U.S. Pat. No. 7,013,887. In said document the heat losses at the end portion of the absorber tube are reduced by means of a double vacuum insulation, resulting from the element connecting to the metal tube, which already provides an insulation and, a second insulation formed between the folding bellows of the expansion compensation device and the glass tube, given that the free space left is very narrow so there is no air exchange. Additionally, at the end of the absorber tube (metal tube) there is placed a protector tube (20) whose axial length matches the length of the bellows, so it can be placed in either the glass tube zone or the glass-to-metal transition zone, that the bellows or expansion compensation devices are not affected by the direct sun radiation.
Basically, the invention deals with an element that allows insulating this zone to decrease thermal losses in addition to avoiding any direct heating due to the incidence of the concentrated light from the mirrors upon the glass-to metal seals.
With this insulation system, it has been observed that when it comes to introducing improvements in the expansion compensation device to increase the solar collector performance, no element addresses the issue of thermal losses at the tubes extremities.
Because of all the foregoing, the object of the present invention is to provide an insulating element that decreases thermal losses at the extremities of the absorber tube to improve the system effectiveness.
That is why, unlike in the known state of the art, our protector element protects from thermal losses a part, which is not the part mentioned in the prior patent (it protects the cover and part of the metal absorber tube, according to proposal 1 or the cover and the vessel according to proposal 2) and to this end it uses a bellow system, which allows to absorb the relative movement between its component parts, something that would be unfeasible with the elements known up to now.